To prevent sewer gases from emanating into a building, every plumbing fixture connected to a sanitary drainage system is protected with a water trap seal. This includes sinks, bathtubs, toilets, etc. Because of its curved shape, a water trap retains a small amount of water after the fixture's use. The water in the trap creates a seal that prevents sewer gas from passing from the drain pipes back out through the fixture (e.g., into the occupied space of a building). If water is not maintained in the trap, then the trap will not serve its function and sewer gases will escape. This can happen, for example, if the plumbing fixture is not used for an extended period of time (e.g., a month or longer), causing the water in the trap to evaporate. This can also happen if siphoning occurs in the trap. Siphoning occurs when a pressure differential within a drain causes the water in the trap to exit the trap.
Many recently built homes include a plumbing outlet box in proximity to an intended location for a washing machine. FIG. 1 illustrates an example plumbing outlet box 10 which includes a back wall 12, and four sidewalls 14-20 that extend outward from the back wall 20 to form a cavity 22 that has an open front. A first one of the sidewalls 14 includes a drain outlet 24 and two liquid inlets 26A, 26B (e.g., one for hot water, and one for cold water). A respective water valve 28A, 28B is secured to each liquid inlet 26. When installed in a building, the cavity 22 is typically recessed into a wall of the building, which enables a washing machine to be placed in close proximity to the wall, and provides convenient access to washing machine plumbing. FIG. 2 illustrates a perspective view of the example plumbing outlet box 10.
The distance between sidewalls 14 and 16 is most commonly 6.25″, while the distance between sidewalls 18 and 20 is most commonly 8″-8.25″. This is the most common configuration found in most new homes. As a lesser used alternative, some alternate plumbing outlet boxes have a distance between sidewalls 14 and 16 of 6.125″-7.5″ and a distance between sidewalls 18 and 20 of 8.75″-9″.
A standpipe 30 is mounted to the drain outlet 24, and connects the drain outlet 24 to a downstream trap 32 that has a water trap seal. A washing machine drainage hose is typically inserted through the drain outlet 24 into the standpipe 30 for washing machine drainage. The drainage hose typically fits loosely into the drain outlet, which permits air to flow freely around the drain tube in the standpipe 30. This has been needed, because if the drain tube and water outlet 24 were to be sealed off, water would be unable to drain through the drain outlet. However, due to the velocity and volume of water being discharged from a washing machine into a washing machine drain (e.g., into standpipe 30), a siphoning effect may occur causing the water level 34 to drop below threshold 36 (or for all water to be lost from the trap). If the water level drops 34 drops below threshold 36, the seal of the water trap 32 is lost, which enables foul-smelling and dangerous downstream sewer gases to escape out of the drain outlet 24 around the drainage hose. Many homeowners and tenants have simply learned to live with having sewer odors in their laundry rooms as a result.